U.S. Pat. No. 6,208,424, for example, describes an interferometer having a light source that emits a beam of rays in the direction of a beam splitter. The beam splitter splits the beam of rays into a measurement beam and a reference beam. The measurement beam then propagates in a measuring arm extending in a first direction between the beam splitter and a measuring reflector. The measuring reflector brings about an offset perpendicular to the direction of incidence between the measurement beam falling on it and the measurement beam reflected back by it. The reference beam propagates in a reference arm extending in a second direction between the beam splitter and a reference reflector, the second direction being oriented perpendicularly to the first direction. In addition, a detector system is provided, to which the superposed and recombined measurement beam and reference beam are able to be supplied, and via which a distance-dependent interference signal with respect to the position of the measuring reflector is able to be generated.
Because of the construction selected for the measuring reflector and the offset produced by it between the beams falling on the measuring reflector and reflected back by it, periodic signal errors otherwise occurring are able to be minimized. Such periodic signal errors may be caused, for example, by non-perfect polarization-optical elements, or else by other optical elements in the paths of the measurement and reference beams.
The measuring reflector described in U.S. Pat. No. 6,208,424 includes two plane mirrors, oriented perpendicularly relative to each other, which bring about the above-mentioned spatial offset of the beams reflected back by them relative to the beams falling on them. Moreover, although it is mentioned that the measuring reflector could also be constructed differently, details concerning an alternative form are not provided.
When working with a form of the measuring reflector having two plane mirrors oriented perpendicularly relative to each other, problems result in the case of a displacement of the measuring reflector along a direction which is oriented orthogonally relative to the measuring direction. In this context, measuring direction is understood to be the direction along which the measuring reflector is movably disposed. In U.S. Pat. No. 6,208,424, x denotes the measuring direction and z denotes the mentioned direction orthogonal to it. For further clarification of these problems, reference is made to FIGS. 1a and 1b. On the left, FIG. 1a shows measuring reflector MR, including two plane mirrors, which is disposed on a machine part MT, and whose position along measuring direction x is to be determined with the aid of the interferometer. FIG. 1a also shows the measurement beam falling twice on measuring reflector MR, the measurement beam falling for the first time being denoted by reference numeral 1, the measurement beam reflected back by measuring reflector MR the first time being denoted by reference numeral 2, the measurement beam falling on measuring reflector MR for the second time being denoted by reference numeral 3, and the measurement beam reflected back by measuring reflector MR the second time being denoted by reference numeral 4. Between the first and second incidence of the measurement beam on measuring reflector MR, the measurement beam impinges on a retroreflector. The right part of FIG. 1a is a top view of measuring reflector MR and the spatial configuration of individual measurement beams 1 to 4, i.e., the resulting rectangular beam cross-section pattern—hereinafter referred to as a spot pattern—in the yz-plane. S denotes the mirror axis of measuring reflector MR oriented along the y-direction, and I denotes the inversion center of the retroreflector. In the case of the provided position of measuring reflector MR along the z-direction, inversion center I is located on mirror axis S of measuring reflector MR.
FIG. 1b shows the conditions when measuring reflector MR, i.e., machine part MT, is displaced upward along the z-direction compared to the position in FIG. 1a. For example, this may be caused by inaccurate guidance. As illustrated in the right part of FIG. 1b, inversion center I of the retroreflector is no longer located on mirror axis S of measuring reflector MR, with the result that before and after the second incidence on measuring reflector MR, measurement beams 3, 4 have a greater distance to mirror axis S than in the case of the first incidence on measuring reflector MR. The result is the spot pattern shown in the right part of FIG. 1b, which is no longer rectangular.
According to the illustrations included in FIGS. 1a and 1b, the resulting spot pattern, that is, the spatial configuration of the measurement beams in the measuring arm in the case of the interferometer described in U.S. Pat. No. 6,208,424, is therefore not invariant with respect to a shift of the measuring reflector in the z-direction, i.e., with respect to a shift along a direction deviating from the measuring direction. The consequence of a spot pattern changing in this manner is that an overall larger construction volume is necessary for the interferometer or the interferometer optical system. The measuring reflector must be dimensioned such that even if it is shifted, the measurement beams all fall on it. Moreover, because of the spot pattern changing in this manner, relatively large beam diameters must be selected in order to ensure a sufficient overlap between the measurement beam and reference beam when they are recombined. Otherwise, the degree of modulation of the distance-dependent interference signals would be too low. Another result of the shift in the measurement beams explained is that they subsequently pass through or impinge on other areas and boundary surfaces of downstream optical components. Unevenness of these components then causes errors in the position measurement.